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(Electronic reproduction permission granted by Elsevier Science Ltd)

Reprinted from:
Atmospheric Environment
Vol. 31, No. 22, pp. 3851-3852 1997
©1997 Elsevier Science Ltd
Printed in Great Britain
1352-2310/97 $17.00+0.00

On July, 16, 1997, the U.S. Environmental Protection Agency (EPA) Administrator signed regulations that will result in the promulgation of new surface ozone and particulate matter standards in the U.S. For surface ozone, EPA will be phasing out and replacing the 1-hour primary standard (maximum hourly average of 0.12 ppm) with a new 8-hour standard, assessed over rolling three-year periods, designed to protect against longer exposure periods. The 1-hour standard will not be revoked in a given area until that area has achieved three consecutive years of air quality data meeting the old standard. The idea is to ensure a smooth, legal, and practical transition to the new longer time base standard. Determination of whether violations of the new standard have occurred will be a much more complex affair than before. The 4th highest 8-hour average daily maximum concentration will be calculated for each year and averaged across an annually-rolling 3-year period, then rounded to the nearest 0.01 ppm. If this value exceeds 0.08 ppm, then it is deemed to be in violation of both the new 'primary' (protection of public health) and 'secondary' (protection of vegetation) standards. The rounding convention used by the Agency will mean that violations will not occur when the 3-year average is less than 0.085 ppm.

There is considerable debate in the United States on whether existing science can justify the level and form of the 8-hour standard. One important issue is determining the 'true' background ozone level in the United States. In the new standards protocol, the EPA identified a range of background levels of 0.03 to 0.05 ppm, and selected 0.04 ppm as the "theoretical" natural background. This, however, is not even in accord with data published by EPA themselves. In Chapter 4 of the EPA document "Air Quality Criteria for Ozone and Related Photochemical Oxidants" (1996), the hourly maximum concentration at all of the remote sites was above 0.040 ppm. Using the data from the sites listed in the document, for 1989, for instance, the 8-hour average daily maximum concentration at Theodore Roosevelt National Park in North Dakota was over 0.07 ppm. In 1992, the top ten 8-hour daily maximum concentrations in Yellowstone National Park, Wyoming ranged from 0.06 to 0.07 ppm. All of the latter occurred in the months of April and May, which would appear to rule out any major impact of long-range transported photochemical pollution.

Similarly, the U.S. Ozone Transport Assessment Group's (OTAG) Air Quality Analysis Workgroup have, in its recent final report, estimated background levels in rural areas surrounding the OTAG region. Although the long-term arithmetic means of the daily maximum 1-hour concentrations were in the range 0.03-0.05 ppm, the top ten 8-hour average daily maximum concentrations were much higher: in the range 0.059-0.090 ppm.

In EPA's assessment of human biological response to ozone, the choice of the natural background value is also important. Whitfield and Richmond, in a paper presented at the 89th Annual Meeting of the Air & Waste Management Association (Nashville, Tennessee, 1996) indicated that human risks could be overestimated by 10% to 37% if the background level was closer to 0.06 ppm than 0.04 ppm. In laboratory experiments on human health effects, researchers used "0" ppm ozone controls; a level rarely attained in the lower atmosphere. It is well known from vegetation experiments that control exposure can seriously affect the level of significance of observed biological results. The same may well apply to human health experiments. If this turns out to be true, then the scientific basis for the selection of 0.08 ppm as the 8-hour ozone standard would be seriously compromised.

Finally, if background ozone levels are indeed closer to 0.06 ppm than 0.04 ppm, then the 'law of diminishing returns' dictates that it will be much more difficult to achieve the legal limit of 0.08 ppm than EPA predicts. I have shown two examples of this in Fig. 1, using sixteen years of hourly ozone data from the EPA's Aerometric Information Retrieval System. In Fig. 1 (a), taking Milwaukee County as a typical example, it can be seen that the greatest rate of reductions in the hourly average concentrations occurs at both the high and low end of the distribution. There has been a greater increase in the 'mid' concentration range. In other words, while it has proved relatively 'easy' to reduce the very highest ozone concentrations, reductions at the 0.08 ppm level have been-and will continue to be-much harder to achieve.

The result of this increasing resistance to improvement can be seen graphically in Fig. 1 (b). Here I have plotted the moving 3-year average of the fourth highest 8-hour daily maximum concentrations (i.e. the new EPA violation criterion) for Fairfield, Connecticut. Although there was a relatively rapid decrease in ozone levels in the early eighties, there is evidence that the trend has slowed in recent years, yet the values still exceed the new EPA standard. All of the empirical evidence, therefore suggests for most sites that presently violate the 8-hour ozone standard, attainment of the new standards may prove elusive.


Figure 1. Changes from 1980 to 1995 in (a) the occurrence of hourly average ozone levels at Milwaukee Co., WI and (b) the 4th highest 8-hour daily maximum ozone concentration at Fairfield, CT.


Allen S. Lefohn, Ph.D.
A.S.L. & Associates
302 North Last Chance Gulch
Suite 410
Helena, Montana 59601, USA

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